The present invention relates to an adaptive modulating method for band compression of video signals, and more particularly to a method and circuit for generating an adaptive constant of an adaptive modulator capable of improving picture quality of the edges of video signals.
A typical adaptive modulator is employed for suppressing channel noise during data transmission when sub-band coding is used as a band compressing method of video signals in a high definition television (HDTV) system, etc. The adaptive modulation technique of this adaptive modulator is proposed for reducing the influence of the channel noise added when a sub-band coded video signal is transmitted in an MIT system such as the HDTV system of America. Moreover, Korean Patent Application No. 91-1023, filed Jan. 22, 1991, by the present inventor discloses a method and circuit for transmitting/receiving video signals using an adaptive modulation technique wherein an input band signal is divided into blocks having given sizes and a maximum value in the blocks is obtained. Next, an adaptive constant is obtained by using an adaptive constant table and the constant is directly multiplied by all the picture elements in the block. The multiplied values are then transmitted through a channel.
However, if there is a picture element A1 of a relatively high amplitude level and picture elements A2 to A6 of relatively low levels as shown in FIG. 1A, a conventional adaptive constant generating circuit does not sufficiently amplify the picture elements of the low level. Specifically, the conventional device will choose a multiplication constant based on all of the picture elements and the high level picture element will have too much of an influence over the choice of a multiplying constant because its amplitude is much higher than the others. As indicated in FIG. 1B, the noise immunity is not enhanced and the circuit is greatly influenced by the channel noise.